Curable adhesive compositions and use thereof

ABSTRACT

A solvent-less hybrid curable composition is prepared from grafting polyesters or polyamides onto a (meth)acrylic copolymer backbone. Besides the many benefits of a solvent-less system, the hybrid curable composition forms strong adhesion to polar substrates, widens the use temperatures, and enables faster processing speeds than conventional hybrid curable compositions. The solvent-less hybrid curable composition forms an optically clear single phase that is suitable as tapes and labels, or in electronic, optoelectronic, OLED and photovoltaic devices, and the like.

FIELD OF THE INVENTION

The present invention relates to curable hybrid acrylic-polyester oracrylic-polyamide adhesives and/or pressure sensitive adhesives. Moreparticularly the hybrid adhesives are optically clear, making theseadhesives particularly well suited for variety of bonding applications.

BACKGROUND OF THE INVENTION

Solvent-based curable adhesives, particularly solvent based pressuresensitive adhesives are widely known. However, solvent based curableadhesives suffer from a number of shortcomings, including longerproduction time, higher energy consumption, difficulties in the controlof production process and final product quality. Moreover, completeremoval of solvent from the solvent-based curable adhesives ischallenging, which can result in emission of solvent vapors andoutgassing issues in the final product and during use. The final productmay not be aligned with current concerns for industry to reduce solventand volatile organic solvents.

Another drawback associated with solvent-based curable adhesives is itsinability to form a thicker coating layer at a single application pass.Multiple passes are typically required to remove solvent for coatingthicknesses greater than 100 microns for solvent-based curableadhesives. Heat is often applied to drive the solvent off of thesolvent-based curable adhesives, yet high application temperature orprolonged heating can lead to premature curing in the applicationequipment.

Solvent-less system is desirable for curable adhesives because it offersa 100% conversion rates with safer and more economical means.

While some solvent-less systems, such as hybrid adhesives, are availableon the market, they have poor performances. Acrylic-polyester basedhybrid copolymers have poor performances and appearances. Polyesterportions of the copolymer can crystallize out of the copolymers, leadingto incompatibility and poor performances as an adhesive.

U.S. Pat. Nos. 4,181,752 and 4,364,972 describe a system wherein acoatable syrup is formed by small degree of prepolymerizing a monomermixture. The syrup contains a large quantity of unreacted monomers,which must be irradiated to undergo polymerization.

U.S. Pat. No. 5,879,759 describes a copolymer formed by first partiallypolymerizing a monomer mixture with irradiation and then addingadditional monomers or oligomers and further irradiating the mixture.The resultant polymer, however, cannot build high molecular weight inthe curing time frames and thus, desirable high performances cannot beachieved.

EP 2548901 describes a radiation curable composition comprising a(meth)acrylic copolymer A and a radiation curable compound B, whereinthe process involves a first copolymerization step and then a subsequentring opening step. One major drawback to this composition isincompatibility of the radiation curable compound B with the(meth)acrylic copolymer A. In addition, the rheology profile of theradiation curable compound B has insufficient pressure sensitivity.

Similarly, U.S. Patent Publication 2013/0251912 describes a mixture of a(meth)acrylic copolymer and a UV curable oligomer. The mixture can leadto immiscibility over time, leading to poor adhesive performances.

There is a need in the art to overcome the disadvantages and limitationsthat exist with solvent-based and curable hybrid copolymer adhesivesand/or pressure sensitive adhesives. The current invention fulfills thisneed.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to solvent-less hybrid curable compositionprepared from grafting polyesters or polyamides onto acrylic copolymerbackbones. The solvent-less hybrid curable composition forms a singlephase that is optically clear. Unlike the conventional linear hybridcurable compositions, the inventive grafted hybrid curable compositionforms strong adhesion to polar substrates, widens use temperatures, andenables faster processing speeds than conventional non-hybridcompositions.

In one embodiment, there is provided a solvent-less, curable compositioncomprising a hybrid copolymer having a meth(acrylic copolymer) backboneand a plurality of a monomer or an oligomer grafted onto themeth(acrylic copolymer) backbone. The monomer or the oligomer comprisesat least one —O— or —NH— functional group.

Another embodiment provides a solvent-less, hybrid curable compositioncomprising a plurality of oligomeric side chains grafted onto a(meth)acrylic copolymer backbone. The (meth)acrylic copolymer backbonecomprises (meth)acrylic monomers and hydroxyl group containing monomers.The oligomeric side chain is a polyester or a polyamide.

Yet another embodiment provides a process for forming a hybrid copolymercomprising the steps of (1) copolymerizing an acrylic copolymer backbonecomprising: (i) a (meth)acrylic monomer, (ii) a hydroxyl groupcontaining monomer and (iii) a cyclic compound having at least onefunctional group, —O— or —NH—, in the cyclic structure; and (2) openingthe cyclic compound with a catalyst, whereby the cyclic compound isgrafted on the acrylic copolymer backbone.

DETAILED DESCRIPTION OF THE INVENTION

All disclosure of all documents cited herein are incorporated in theirentireties by reference.

Acrylic copolymers are incompatible with polyesters and polyamides, andwhen combined, they form a hazy appearance because one of the copolymerscrystallize out of the mixture. Unlike the mixture, the solvent-lesshybrid curable composition is completely miscible and forms a singlephase without any separation or haziness. The hybrid may be used foroptically clear applications and has superior adhesive performances thanconventional mixtures of acrylic and other polymer systems.

The solvent-less, curable hybrid copolymer is prepared from a(meth)acrylic copolymer backbone and a plurality of monomers oroligomers that comprises an —O— or —NH— functional group. The backboneof the curable hybrid copolymer is formed by a meth(acrylic monomer)(A1) and a copolymerizable hydroxyl group containing monomer (A2).

The term “(meth)acryl” is to be understood as to encompass both theterms “acryl” and “methacryl” and refers to compounds comprising atleast one acrylate group (CH2═CHCOO—) and/or at least one methacrylategroup (CH2═CCH3COO—). The alkyl(meth)acrylates are preferably selectedfrom linear and branched aliphatic alkyl(meth)acrylates.

The (meth)acrylic monomer (A1) is generally used in an amount from about30 to about 99 wt % based on the sum of the hybrid copolymer mixture.The amount of (meth)acrylic monomer (A1) is preferably at least about 40wt %, more preferably at least 50 wt %, preferably up to about 95 wt %,more preferably up to about 90 wt % based on the sum of the hybridcopolymer mixture.

The alkyl(meth)acrylates are preferably selected from linear andbranched aliphatic alkyl(meth)acrylates, more preferably from thosehaving from 3 to 20 carbon atoms in the alkyl group. Particularlypreferred are methyl methacrylate, n-butylacrylate, iso-butylacrylate,2-ethyl hexylacrylate, isobutyl methacrylate, methoxy polyethyleneglycol acrylate (average Mn 350), and mixtures thereof.

The copolymerizable hydroxyl group containing monomer (A2) is generallyused in an amount of about 1 to about 30 wt % based on the sum of thehybrid copolymer mixture. The amount of copolymerizable hydroxyl groupcontaining monomer (A2) used is preferably at least about 1 wt %, morepreferably at least about 5 wt %, and up to about 30 wt %, and morepreferably up to about 20 wt % based on the sum of the hybrid copolymermixture.

The copolymerizable hydroxyl group containing monomer (A2) contains atleast one hydroxyl functional group that can react with the cycliccompound (B) during the ring opening reaction step. The copolymerizablehydroxyl group containing monomer contains a hydroxyl group either in anaromatic or an aliphatic form. Preferred hydroxyl group containingmonomer (A2) include hydroxyalkyl(meth)acrylates, the ethoxylated and/orpropoxylated derivatives thereof, the adducts thereof with lactones,polyalkoxy monohydroxy mono(meth)acrylates. Particularly preferred arehydroxyalkyl(meth)acrylates having from 1 to 20 carbon atoms in thealkyl group, the ethoxylated and/or propoxylated derivatives thereof,the adducts thereof with lactones, polyalkoxy monohydroxymono(meth)acrylates. Examples of such compounds comprisehydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate,hydroxybutyl(meth)acrylate, hydroxypentyl(meth)acrylate,hydroxyheptyl(meth)acrylate, hydroxynonyl(meth)acrylate,hydroxydecyl(meth)acrylate, their position isomers, the ethoxylatedand/or propoxylated derivatives thereof, the adducts thereof withlactones, diethylene glycol mono(meth)acrylate, polyethylene glycolmono(meth)acrylate, propylene glycol mono(meth)acrylate, polypropyleneglycol mono(meth)acrylate.

Another preferred hydroxyl group containing monomer (A2) include phenolgroups, such as 4-vinylphenol.

Another preferred hydroxyl group containing monomer (A2) are thosecontaining carboxylic acid groups, and mixtures of any of them. Examplesof such compounds are (meth)acrylic acid, β-carboxyethyl(meth)acrylate,crotonic acid, maleic acid, fumaric acid, itaconic acid. Mixtures of anyof the above copolymerizable monomers can be used. More preferredcopolymerizable monomers are 2-hydroxyethyl(meth)acrylate,2-hydroxybutyl(meth)acrylate, 4-hydroxybutyl(meth)acrylate, and mixturesthereof. Most preferred are 2-hydroxyethylacrylate,2-hydroxybutylacrylate, 4-hydroxybutylacrylate, and mixtures thereof.

Optionally, the backbone may further comprises additionalcopolymerizable monomers. The optional copolymerizable monomer (A3) maybe added up to 60 wt % of the backbone, preferably up to 50 wt %, morepreferably up to 40 wt % based on the sum of the hybrid copolymermixture. The copolymerizable monomer (A3) is generally a compoundcontaining at least one copolymerizable carbon-carbon double bond.Copolymerizable carbon-carbon double bonds are known to the skilledperson and include (meth)acrylate, vinyl, allyl types of double bonds.Suitable copolymerizable monomers (A3) are linear and branched aliphaticalkyl(meth)acrylates, especially those having from 1 to 20 carbon atomsin the alkyl group, glycidyl(meth)acrylate, vinyl acetate, styrene andmixtures thereof. Particularly preferred are methyl(meth)acrylate,ethyl(meth)acrylate, tert-butyl(meth)acrylate, vinyl acetate, styreneand mixtures thereof.

In another embodiment, the optional copolymerizable monomer contains aphotoinitiator group in the monomer, in an amount of up to about 10 wt%, and preferably up to about 3 wt %, based on the sum of the hybridcopolymer mixture. Exemplary copolymzerizable monomers containingphotoinitiator groups include 2-(4-benzyol-3-hydroxyphenoxy)ethylacrylate, benzoin acrylate,2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-propanone,4-acryloyloxy benzophenone, and mixture of 4-acryloyloxyoligoethylenoxycarbonyl-benzophenones. Also preferred photoinitiator containingcoolymerizable monomers include those taught in EP 167870.

The backbone is formed by copolymerizing the (meth)acrylic monomer (A1)and the copolymerizable hydroxyl group containing monomer (A2). Thecopolymer may be a random, alternating or block copolymer. It ispreferably a random copolymer. The copolymerization in thecopolymerization step may take place by free-radical copolymerization.This may take place in a manner known to the skilled person byconventional methods, particularly by free-radical polymerization usingthermal radical initiators. Examples of suitable thermal radicalinitiators include peroxides, such as benzoyl peroxide, azo compounds,such as azo-bis-isobutyronitrile, azo-bis-dimethylpentanenitrile,azo-bis-methylbutyronitrile, azo-bis-cyanocyclohexane. The initiatorsmay be used, for example, in amounts from 0.05 to 2.0 wt % based on thesum of the hybrid copolymer mixture.

To achieve a good control of the molecular weight and its distribution,a chain transfer agent, preferably of the mercaptan type, such asn-dodecylmercaptan, tert-dodecanethiol, iso-octylmercaptan,n-octylmercaptan or of the carbon halide type, such as carbontetrabromide, bromotrichloromethane, can also be added in the course ofthe reaction. The chain transfer agent is generally used in amounts ofup to 5 wt % based on the sum of the hybrid copolymer mixture.

The copolymerization is generally carried out at a temperature from 60to 150° C., preferably under an inert gas atmosphere. Thecopolymerization is preferably carried out at a temperature from 60 to100° C.

The backbone copolymerization is followed by a subsequent step ofopening the cyclic compound. The ring opening step grafts the cycliccompound B onto the (meth)acrylic polymer backbone. The cyclic compoundis grafted at the hydroxyl group of the (meth)acrylic polymer backbone.The cyclic compound having at least one functional group X in the cyclicstructure, where X is —O— or —NH—. The amount of cyclic compound havinga functional group (B) used is preferably at least 5 wt %, morepreferably at least 10 wt %, preferably up to 30 wt %, more preferablyup to 60 wt %, based on the sum of the hybrid copolymer mixture.Preferred cyclic compounds include lactones, lactams, lactides, cycliccarbonates and mixtures thereof. Preferred cyclic compounds B arelactones and lactides and mixtures thereof. Particularly preferred arelactones such as L(−)lactide, ε-caprolactone, δ-valerolactone,γ-butyrolactone, and lactones of hydroxycarboxylic acids such as2-hydroxycarboxylic acids, e.g. glycolic acid and lactic acid,3-hydroxycarboxylic acids, e.g. 3-hydroxypropionic acid,3-hydroxybutyric acid, 3-hydroxyvaleric acid and hydroxypivalic acid.More preferred are ε-caprolactone, δ-valerolactone, γ-butyrolactone andmixtures thereof, most preferred is ε-caprolactone.

The ring opening step is generally carried out at room temperature to upto about 150° C. The ring opening reaction can take place without theuse of a catalyst, but the reaction rate can be increased with theaddition of the catalyst. Therefore, the ring opening reactionpreferably takes place in the presence of at least one catalyst.Suitable catalysts include alkali or alkaline earth metal alkoxides,organic acids, inorganic acids and Lewis acids such as sodium methoxide,calcium methoxide, aluminum isopropoxide, tetraalkyl titanates, titaniumchelates, titanium acylates, lead salts, lead oxides, zinc borate,antimony oxide, stannous octoate, tin laurate, tin octoate, dibutyl tindilurate, sulfuric acid, hydrochloric acid, phosphoric acid, borontrifluoride. Another preferred catalyst include yttrium alkoxides andlanthanum alkoxides, both of which can be used to carry out the ringopening step at room temperature. The catalyst can be used in amounts ofup to 1% based on the sum of the hybrid copolymer mixture.

The resultant hybrid copolymer contains a random copolymer of(meth)acrylic copolymers and a plurality oligomeric arms grafted ontothe backbone. The backbone contains hydroxyl or amine terminalfunctional groups in the oligomeric arms.

The hybrid curable copolymer can be used as is, or can be furtherformulated and mixed with other components.

Additional monomers such as UV-curable monomer and UV-curable resin canbe added to the hybrid curable copolymer. The mixture can then beirradiated to crosslink the mixture. In another embodiment, the hybridcurable copolymer backbone can be thermally cured with the addition ofisocyanate and/or melamine crosslinkers.

Optionally, tackifier, photointiator, stabilizer, viscosity modifiersmay be added to the hybrid curable copolymer. They may be added inamounts known in the art to impart specific performances to the curedcomposition.

The crosslinked hybrid composition forms strong bonds with polarsubstrates, and particularly with PVC, glass, polyester polyurethanefoam, and the like. The grafted polyester or polyamide side chains ofthe crosslinked (meth)acrylic polymer forms hydrogen bonds with polarsubstrates and stronger adhesion is formed.

The crosslinked hybrid composition is also optically clear and remainsin a single phase. Polyesters or polyamide in the hybrid compositiondoes not crystalize out of the hybrid polymer. Due to the opticallyclear properties, the hybrid composition is suitable in manyapplications such as labels or tapes; electronics, optoelectronics,OLEDs, photovoltaic devices; and the like. Because the hybridcomposition remains in a single phase, and peel, tack and shearperformances remain superior, especially on polar substrates.

The hybrid composition also has wider use temperature since theviscosity of the grafted hybrid composition is lower than thenon-hybrid, linear-chain copolymer at the same temperature. Somebenefits associated with lower viscosity of the hybrid compositionsinclude faster coating speeds, faster processing speeds, easierfiltration, lower thermal degradation and better levering of the coatedsubstrate surfaces. Moreover, the hybrid composition can be manufacturedand coated at lower temperatures, which decreases overall carbonfootprint.

Many modifications and variations of this invention can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. The specific embodiments described herein areoffered by way of example only, and the invention is to be limited onlyby the terms of the appended claims, along with the full scope ofequivalents to which such claims are entitled.

EXAMPLES

Peel strength was measured by performing a 180° peel test on stainlesssteel panels using an Instron. About 1.0 mil thick coating of theadhesive was applied on to silicone release paper using heated rollersand bonded to Mylar film. Three specimens each 1″ by 1″ in dimensionswere cut perpendicular to the machine direction from the coated Mylar.After conditioning overnight at 72° F. and 50% relative humidity, therelease paper was removed and the specimens are bonded to stainlesssteel panels. The bonds are then rolled using a 4.5 lb. roller. Afterconditioning the bonds for about 20 minutes, the bonds were peeled inthe Instron at 2″/minute. The stainless steel panel was in thestationary jaw, and the Mylar was in the movable jaw. The results arereported as an average load in oz/in. This test was repeated afterconditioning for 24 hours.

For shear measurement, about 1.0 mil thick coating of the adhesive wasapplied on to silicone release paper using heated rollers and bonded toMylar film. The lamination was cut to 1 by ½ inch strips. The releasepaper is removed and the Mylar bonded to stainless steel. A one miltransfer coating of the adhesive was made using heated rollers and a 1.0by 0.5 inch strip of the specimen was bonded to a stainless steel plate.The bonds were then rolled using a 4.5 lb roller. After conditioning thebonds for about five hours, the plates are secured onto the shear teststand a 1000 gram weight is applied to each plate. The time that thespecimen takes to slip from the plate was recorded as shear strength.

Solvent-less, curable hybrid compositions were made in accordance withthe following components and processes and irradiated with UV radiationto form cured hybrid compositions.

Example 1

In a one-liter, four-neck glass reaction flask equipped with a stirrer,a water cooled condenser and an inlet for nitrogen and a thermoprobemounted in the flask which is attached to a thermoregulator for masstemperature control, Solution 1 was charged to the flask and the contentwas heated to 70° C. The temperature was kept at 70° C. for 60 minutesunder continuous agitation. The nitrogen purged through the flask at amoderate rate. The Solution 2 was then added to the reaction vessel over80 minutes period while the temperature was continuously kept at 70° C.After the addition of the Solution 3, the reaction was kept at 70° C.for 60 minutes and then at 80° C. for another 60 minutes. The solutionwas then charged to the flask under agitation and the reaction wascarried at 115° C. for 5 hours.

Solution 1 Amounts (g) caprolactone 80.0 2-ethylhexyl acrylate and/or104.0 Isobutyl methacrylate 2-hydroxethyl acrylate 12.0 photoinitiator0.1 Chain transfer agent 0.4 Initiator 0.4

Solution 2 Amounts (g) 2-ethylhexyl acrylate and/or Isobutyl 104.0methacrylate methoxy polyethylene glycol acrylate 2.0 (350 mw),2-hydroxethyl acrylate 12 photoinitiator 0.7 Chain transfer agent 0.7initiator 0.4

Solution 3 Amounts (g) Hydroxyl butyl acrylate 9.8 stabilizer 0.15catalyst 0.015

The final polymer was a colorless and optically clear, without anyseparation of phases.

A 25 microns thick coating was first coated onto a PET film and thencured under H-bulb (Fusion System) with 40 mJ/cm2 UVC radiation. Thecured coating had the following properties:

-   -   (a) 20 min peel on stainless steel panel: about 41.1 oz/inch    -   (b) 24 h peel on stainless steel panel: about 44.0 oz/inch    -   (c) Shear at room temperature (4.4 psi): about 4.8 hours

The cured hybrid copolymer, when applied as coating was clean,water-white and remained clear at room temperature for over 6 months.The cured hybrid copolymer had good adhesion properties as adhesivecoatings.

Example 2

In a one-liter, four-neck glass reaction flask equipped with a stirrer,a water cooled condenser and an inlet for nitrogen and a thermoprobemounted in the flask which is attached to a thermoregulator for masstemperature control, Solution 1 was charged to the flask and the contentwas heated to 70° C. The Solution 2 was then added to the reactionvessel over 180 minutes period while the temperature was continuouslykept at 70° C. After the addition of the Solution 2, the reaction waskept at 70° C. for 60 minutes and then at 80° C. for another 60 minutes.Following the 60 minutes hold, Solution 3 was charged to the reactionmixture under agitation and the reaction was carried at 105° C. foreight hours to finish the reaction.

Solution 1 Amounts (g) Caprolactone 56.00 Initiator 0.28

Solution 2 Amounts (g) One or more selected from: 2-ethylhexyl 286.72acrylate, Isobutyl methacrylate, hydroxyl butyl acrylate 50.40Caprolactone 67.20 Chain transfer agent 2.04 Initiator 1.48

Components to Solution 3 Amounts (g) Viscosity modifier 21.00 Stabilizer0.19 Catalyst 0.07

One hundred parts of the reaction products were then formulated with 5parts crosslinking monomer, 5 parts tackifying monomer, 10 partsviscosity modifier, and 0.3 parts photoinitiator.

A 175 microns thick coating coated onto a PET substrate and then wascured under H-bulb (Fusion System) with 950 mJ/cm2 UVA radiation.

The cured product was colorless and optically clear, without anyseparation of phases. The cured coating also had the followingproperties:

-   -   (a) 20 min peel on stainless steel panel: about 23.6 oz/inch;        and    -   (b) 24 h peel on stainless steel panel: about 24.4 oz/inch.

Even with a thickness that is seven-folds greater than a typicaladhesive coating, the cured product had good peel properties. Theadhesive coating was then aged under each of the following conditions:

-   -   (1) Thermal aging in 85° C. oven for 500 hours;    -   (2) Weathering oven (60° C. and 95% RH) for 500 hours; or    -   (3) Accelerated UV radiation aging (QUV) for 500 hours.        All the adhesives remained colorless and optically clear,        without any phase separation.

We claim:
 1. A process forming a hybrid copolymer comprising the stepsof: a) copolymerizing an acrylic copolymer backbone comprising: (i) a(meth)acrylic monomer, (ii) a hydroxyl group containing monomer and(iii) a cyclic compound having a polyester in the cyclic structure, (iv)a third (meth)acrylic monomer that contains a photoinitiator group b)opening the cyclic compound with a catalyst, whereby the cyclic compoundis grafted on the acrylic copolymer backbone wherein the process issolvent-free.
 2. The process of claim 1 wherein the hybrid copolymercomprises: (1) about 30 to about 99 wt % of the (meth)acrylic monomer;(2) about 1 to about 30 wt % of the hydroxyl group containing monomer;(3) about 5 to about 60 wt % of the cyclic compound; and (4) up to about10 wt % of the third (meth)acrylic monomer; wherein the total wt % is100 wt %.
 3. The process of claim 1 further comprising the steps of: (c)adding a UV-curable monomer, UV-curable resin, tackifier, plasticizer,photointiator, cross-linker, chain transfer reagent, and/or stabilizer;and (d) irradiating the hybrid copolymer; whereby a cured hybridcopolymer is formed.
 4. The process of claim 2 further comprising thesteps of: (c) adding an isocyanate monomer, melamine crosslinker,tackifier, plasticizer, cross-linker, chain transfer reagent, and/orstabilizer; and (d) thermally curing the hybrid copolymer; whereby acured hybrid copolymer is formed.